1. Field of the Invention
The present invention concerns a radio-frequency antenna for a magnetic resonance system of the type having a number of antenna rods and two rings, with the antenna rods disposed regularly around an antenna axis and each connected at their respective rod ends with one of the rings and wherein either each antenna rod proceeds substantially parallel to the antenna axis and exhibits a rod spacing in a middle region of the antenna axis, or each antenna rod forms with the antenna axis an inclination angle, such that the radio-frequency antenna is in the shape of a frustrum and, at its rod end situated farther from the antenna axis, exhibits a rod spacing from the antenna axis.
2. Description of the Prior Art
Radio-frequency antennas of the above type are—in particular in the case of the antenna rods being parallel to the antenna axis—generally known as birdcage resonators. Capacitors are arranged in the rings and/or the antenna rods. The radio-frequency antenna is adjusted so that, for a predetermined or predeterminable operating frequency of the radio-frequency antenna, it forms a resonant oscillating circuit.
Birdcage resonators are known from German PS 197 32 783 and Patent Abstracts of Japan for JP-A-2000 166 895, in which the rings are farther from the antenna axis than the antenna rods.
Nuclear spins of an examination subject (often a person) are excited to resonance by magnetic fields emitted by such radio-frequency antenna. The resonances are then acquired and evaluated. The acquisition of the resonance signals can ensue with the same antenna.
Due to the trend toward ever-shorter magnet systems, the risk exists in conventional whole-body transmitting antennas that are image artifact known as the ambiguity artifact will occur. Ambiguity artifacts are caused by, due to the superimposition of basic magnetic field and gradient magnetic field, two or more locations existing at which the same total field strength predominates. Typically, one of these locations is situated within a desired acquisition volume, and at least one further location is situated outside of the acquisition volume. If the radio-frequency field strength at the location outside of the acquisition volume is of a sufficient volume, it leads to the superimposition of the image information from both locations in the image reconstruction.
To prevent the ambiguity artifact, it must be insured that the radio-frequency magnetic field outside of the acquisition volume decreases sufficiently rapidly. It must simultaneously be insured for that the radio-frequency signal is sufficiently homogenous within the acquisition volume.
Whole-body transmitting antennas normally are fashioned as birdcage resonators. Due to this design, these antennas already possess a rapid field drop off in the lengthwise (longitudinal) axis, however, severe limitations in the field homogeneity arise in the case of very short magnets and/or very short antennas.
From the scientific essay “Fast drop off cylindrical Radio-frequency transmit coils”, by E. B. Boskamp, appearing in the Proceedings of the ISMRM volume 10 (2002), a combination of a number of birdcage resonators is specified that are connected with one another such that a suitable field profile can be generated.
A radio-frequency antenna for a magnetic resonance system is known from U.S. Pat. No. 6,344,745 that has a number of antenna rods and two rings. In this radio-frequency antenna, the antenna rods are regularly arranged around an antenna axis and are connected at their rod ends with one of the rings per rod end. Each antenna rod is substantially parallel to the antenna axis and exhibits in a middle region a rod spacing from the antenna axis that is larger than a ring spacing from the antenna axis for at least one of the rings in the region of this antenna rod. In this radio-frequency antenna, the antenna rods gradually proceeds radially inwardly over an area that amounts to at least 37.5% of the total length of the antenna rods. In this area, the antenna rods are curved radially inwardly in the shape of a circular arc. This area extends to the end of the respective antenna rod.
A radio-frequency antenna for a magnetic resonance system is known from German OS 100 52 192 that has a number of antenna rods and two rings. In this radio-frequency antenna, the antenna rods also are regularly arranged around an antenna axis and also are connected at their rod ends with one of the rings per rod end. The antenna rods also are substantially parallel to the antenna axis and exhibit in a middle region a rod spacing from the antenna axis that is larger than a ring spacing from the antenna axis for at least one of the rings in the region of this antenna rod. This is achieved by the antenna rods being curved over their entire length, such that the radio-frequency antenna has a barrel-like appearance.
Various radio-frequency antennas for a magnetic resonance system are known from Japanese Application 2001 145 608. In these radio-frequency antennas, a number of antenna rods are regularly arranged around an antenna axis. The antenna rods are connected at their rod ends with rings. In one of these radio-frequency antennas, the antenna rods, relative to the antenna axis, form an inclination angle, such that the radio-frequency antenna is fashioned in the shape of a frustrum. At the rod ends situated farther from the antenna axis, the antenna rods exhibit a rod spacing from the antenna axis. At this location, the ring appears to exhibit the same spacing from the antenna axis as the respective antenna rod.
A radio-frequency antenna for a magnetic resonance system is known from U.S. Pat. No. 4,736,161 that has a number of antenna rods and two rings. In this radio-frequency antenna, exactly two detuning circuits are present by means of which the radio-frequency antenna can be detuned. The detuning circuits are connected to supply cables for the radio-frequency antenna. A similar arrangement is described in European Application 0 758 091.
A radio-frequency antenna for a magnetic resonance system is known from U.S. Patent Application Publication No. 2002/0011843 that has a number of antenna rods and two rings, wherein the antenna rods form two substructures twisted opposite one another, each of the substructures being regularly arranged around the antenna axis, and the substructures exhibiting rod spacings differing from one another. The substructures are thereby arranged in a circular fashion around the antenna axis.